Biochemical and histochemical studies were made, with the following results, on the metabolic changes in tumor, using young chickens inoculated with chicken sarcoma virus. 1. The metabolic changes in the process of tumor development were observed to occur in conformity with the following three stages : the stage of stimulation by foreign body, the precancerous growth and the cancerous growth stages. Each of these three stages presented its characteristic metabolic features. 2. The metabolic features in the tumorous growth stage were found to be characterized by an increase in anaerobic glycolysis, partial inhibition of oxydative respiration and increase both in DNA and in RNA. These metabolic features of tumor were not identical to those observed in regenerating tissues. 3. The tumorous growth was observed as early as a few days after virus inoculation, contrary to the findings obtained with any other carcinogenic chemicals. 4. No determination could be made as to which of these metabolic features is really due to the propagation of the virus and which is due to the tumorous growth itself
